Apple scab is of major economic importance in the areas where apples are grown. If not controlled, the disease can cause extensive losses (70 percent or greater) where humid, cool weather occurs during the spring months. Losses result directly from fruit or pedicel infections, or indirectly from repeated defoliation which can reduce tree growth and yield.
Apple scab (see FIG. 1) can be observed on leaves, petioles, blossoms, sepals, fruit, pedicels, and less frequently, on young shoots and bud scales. The first lesions are often found on the lower surfaces of leaves as they emerge and are exposed to infection in the spring. Later, as the leaves unfold, both surfaces are exposed and can become infected. Young lesions are velvety brown to olive green and have feathery, indistinct margins. With time, the margins become distinct, but they may be obscured if several lesions coalesce. As an infected leaf ages, the tissues adjacent to the lesion thicken, and the leaf surface becomes deformed. Young leaves may become curled, dwarfed, and distorted when infections are numerous. The lesions may remain on the upper and lower leaf surface for the entire growing season; occasionally, the underlying cells turn brown and die, so that brown lesions are visible on both surfaces. The number of lesions per leaf may range from one or two to more than a hundred. The term “sheet scab” is often used to refer to leaves with their entire surfaces covered with scab. Young leaves with sheet scab often shrivel and fall from the tree. Infections of petioles and pedicels result in premature abscission of leaves and fruit, respectively. In late summer or early fall, lesions may appear whitish due to the growth of a secondary fungus on the lesion surface.
Lesions on young fruit appear similar to those on leaves, but as the infected fruit enlarge, the lesions become brown and corky. Infections early in the season can cause fruit to develop unevenly as uninfected portions continue to grow. Cracks then appear in the skin and flesh, or the fruit may become deformed. The entire fruit surface is susceptible to infection, but infections early in the season are generally clustered around the calyx end. Fruit infections that occur in late summer or early fall may not be visible until the fruit are in storage. This symptom is called “pin-point” scab, with rough circular black lesions ranging from 0.004 to 0.16 inch (0.1-4 mm) in diameter.
Although research in New York has shown that the scab fungus can overwinter in trees as conidia on bud scales, the pathogen generally overwinters in leaves and fruit on the orchard floor. Ascospores are the major source of primary inoculum and are produced within pseudothecia that develop in leaves during the winter months. In a typical year in most locations, the first mature ascospores are capable of causing infections at about the time of bud break or soon thereafter. Ascospores continue to mature and are discharged over a period of five to nine weeks, with peak discharge during the pink to petal fall phenological stages. The length of time required for infection to occur depends on the number of hours of continuous wetness on the leaves and the temperature during the wet period. Young leaves remain susceptible for five to eight days, but their lower surfaces may become infected in late summer. For fruit, the duration of the wet period required for infection increases with the age of the fruit, which remains susceptible until harvest. Once the fungus is established in the leaf or fruit, conidia form on the surface of the lesion and become the source of secondary inoculum for the remainder of the season. Conidia are disseminated to developing leaves and fruit by splashing rain and wind. Several secondary cycles of conidial infection may occur during the growing season depending upon the frequency of infection periods and the susceptibility of host tissue.
Management of apple scab is multifaceted, with resistant cultivars, sanitation, and chemicals all being used to some degree depending on the orchard system being used and the goals of the grower.
Most of the major apple cultivars are susceptible to the fungus, although this varies somewhat. More than 25 scab-resistant cultivars have been released, included Prima, Priscilla, Jonafree, Redfree, Liberty, Freedom, Goldrush, and Pristine. Most are adapted to the more northern apple-growing areas of the U.S. All scab-resistant cultivars vary in their susceptibility to other early-season diseases; and all are susceptible to the summer diseases. Some recently released apple cultivars that have not been bred specifically for resistance to scab show varying levels of scab susceptibility, also.
Prevention of pseudothecial formation in overwintering apple leaves would probably eliminate scab as a serious threat to apple production. Unfortunately, complete elimination of pseudothecia is not possible under orchard conditions with current methods.
Apple scab is controlled primarily with fungicide sprays. A variety of fungicide sprays with differing modes of action are available. When and how they are used depends upon their mode of action. Protectant fungicides prevent the spores from germinating or penetrating leaf tissue. To be effective, they must be applied to the surface of susceptible tissue before infection occurs. Occurrence of infection can, amongst others, be predicted with an accurate weather forecast. Protectant fungicides are applied routinely at 7 to 10 day intervals or according to anticipated infection periods.
Post infection fungicides control the scab fungus inside leaves and fruit. These chemicals can penetrate plant tissues to eliminate or inhibit lesion development. The ability of these fungicides to stop infections is limited to a few hours, or up to few days (depending upon the specific fungicide), and their effect often varies according to temperatures during the first 24 to 48 hours after infection. Some fungicides can inhibit the fungus even later into the incubation period (the time between infection and the appearance of symptoms). Eradication of scab lesions after they appear does not usually occur, but can be achieved with the proper rate and timing of certain fungicides. The selection of fungicides for management of scab is based on several factors, including the entire spectrum of other diseases that must be managed at that time, the potential for resistance in the scab fungus to the selected chemical, the history of the disease in a particular orchard, the final market for the fruit, and other social and economic factors. Good horticultural practices, such as proper site selection, tree spacing and annual pruning, facilitates better chemical control by improving spray coverage and reducing the length of wet periods. Chemical fungicides used in the treatment or prevention of apple scab include maneb, mancozeb, captan, pyrimethanil and tolylfluanide.
There is thus still need for a natural fungicide, which is effective for the control of apple scab, and which is environment-friendly and non-toxic for humans and/or animals.